The invention relates to an image forming apparatus, such as an electrophotographic copying machine, equipped with a transferring belt device for electrostatically supporting and conveying a transfer material to an image carrying member and for further conveying the transfer material to a fixing means after a toner image on the image carrying member has been transferred onto the transfer material.
FIG. 7 is a sectional views showing an electrophotographic copying machine as one example of a conventional image forming apparatus. In the drawing, numeral 10 indicates a photoreceptor drum, carrying means, and numeral 20 indicates a charging device for charging the circumferential surface of photoreceptor drum 10. There are further provided exposure device 30, image exposure station 13, developing device 40, transfer sheet feeding device 50, sheet feeding roller 53, transferring device 16 which is a charging device used for transferring, separating device 17 which is a charge eliminating device for separation, fixing device 80, and cleaning device 90. The mark P represents a recording sheet as a transfer material. Numeral 56 indicates a conveying belt as conveying means for recording sheet P.
This copying machine works as follows. After charging device 20 charges evenly the circumferential surface of photoreceptor drum 10, exposure device 30 exposes the circumferential surface of photoreceptor drum 10 at image exposure station 13 to form an electrostatic latent image on it. Thereafter, the latent image is developed to form a toner image. The toner image is transferred, with controlled timing, onto recording sheet P which is fed by sheet feeding roller 53 of transfer sheet feeding device 50. This transferring operation is carried out in such a manner that transferring device 16 charges the reverse side of recording sheet P with the polarity opposite to the charged polarity of the toner. After this transferring operation, recording sheet P is neutralized by the application of high A.C. voltage by separating device 17 so that recording sheet P is separated from photoreceptor drum 10. The separated recording sheet P is conveyed to a fixing device by conveying belt 56 to fix the toner image, and then discharged to a delivery tray outside of the machine.
After transfer of the toner image, toner remaining on the circumferential surface of photoreceptor drum 10 is cleaned by cleaning device 90, and then photoreceptor drum 10 is made ready for the next copying operation.
However, in the above constitution, in order to ensure the transferability and separability of the toner image, the best working point needs to be set by the adjustment of electric discharge efficiency between transferring device 16 and separating device 17. Further, there is a problem in that, as the electric discharge efficiency is influenced by the environment, the allowable range for reliability is too narrow. The transferability depends on the charges held on recording sheet P and the mechanical characteristics of recording sheet P, such as stiffness, surface smoothness and the extent of curl orientation. Since these factors are greatly influenced by the storage conditions of recording sheet P and the environment (temperature, humidity) at the time of transferring, it may be difficult to maintain good conditions at all times.
Furthermore, when recording sheet P moves to conveying belt 56, the recording sheet is returned into it's original flat condition. When the diameter of photoreceptor drum 10 is fairly large, curvature of recording sheet P is smaller than when the photoreceptor diameter is small. This curved recording sheet P onto which a toner image has been transferred, is apt to adhere onto photoreceptor drum 10, so that sheet jamming may often take place. Accordingly, this conventional type image forming apparatus has a disadvantage in that the sheet passing capability of its mechanism may be low.
As a technique which has improved the above disadvantage, U.S. Pat. No. 3,357,325 discloses a belt type transfer conveyance apparatus as shown in FIG. 8.
In FIG. 8, numeral 10 indicates a photoreceptor drum as a drum-shaped image carrying member, 70a a transferring belt device as a belt-shaped transfer and convey means, 71 a transferring belt for attracting a transfer material by its holding charges, 72 and 73 are rollers for supporting, stretching and rotating the transferring belt 71, 60 a pretransfer sheet charging device for applying a corona discharge to electrostatically attract recording sheet P onto transferring belt 71, 16 a transferring device as a charging device for transferring a toner image from photoreceptor drum 10 onto recording sheet P, 160 a transferring point. With this constitution, recording sheet P can be conveyed by being attracted onto transferring belt 71 so that excellent transfer and separation efficiencies may be obtained at transferring point 160.
Transferring belt device 70a may be a preferable device for a color image forming apparatus in which toner images are formed by being superimposed on photoreceptor 10 and are transferred at one time onto recording sheet P. In a color image forming apparatus, in order to superimpose toner images on photoreceptor 10, a plurality of developing devices need to be installed around the periphery of photoreceptor drum 10, and thereby the drum diameter becomes inevitably larger. Accordingly, since a sufficient separation performance may not be obtained by a conventional electrostatical transfer-separation method, the apparatus needs an enhanced method to ensure separation performance. Furthermore, since the adhering amount of toner may be increased so that a large amounts of transferring charge may be required, the apparatus needs to have the capability of holding the large amounts of transferring charge. For the above two requirements the transferring belt device performs well.
In transferring belt device 70a shown in FIG. 8, recording sheet P is charged by pre-transfer sheet charging device 60 before transferring, and then is conveyed by running transferring belt 71, being attracted thereto. A charging device of a corona-discharge type has conventionally been used as aforesaid pre-transfer sheet charging device 60.
In a charging device of a corona-discharge type, however, when a recording sheet enters below discharging electrode 62 without being attracted fully to transferring belt 71, it has caused problems that the recording sheet is not charged fully, the recording sheet can not be separated fully from photoreceptor drum 10, and the leading edge of recording sheet P curves and advances toward discharging electrode 62 of pretransfer sheet charging device 60, resulting in the snapping of a wire and a short circuit. Even in the case of the technology disclosed in Japanese Patent Publication Open to Public Inspection Nos. 121878/1989 and 121879/1989 (hereinafter referred to as Japanese Patent O.P.I. Publication), a recording sheet is charged through corona discharge and therefore the aforesaid problems are not solved. In order to solve the problems, there is a method wherein a plate member is provided at the upstream side of shielding member 61 so that recording sheet P can advance properly, which, however, is not satisfactory because charging in this method is not uniform.
Further, discharging electrode 62 composed of a thin wire tends to be contaminated while it is used, which causes uneven discharging that results in uneven charges given to recording sheet P. When transferring is made under such condition, uneven transferring corresponding to uneven discharging of pre-transfer sheet charging device 60 is created, which has been a problem.
In reversal development, which has been recently widely used, the use of charging with negative polarity has increased. In this type, when negative polarity charging is performed by corona charging, recording sheets are apt to be charged unevenly, resulting in the above problems.
Further, the force for recording sheet P to be attracted to transferring belt 71 is reduced by transfer charging made for transferred toner images, and the recording sheet tends not to be separated from photoreceptor drum 10, which has been a problem.
Further, in the conventional apparatus, ozone is generated due to corona discharge, thus deteriorating transferring belt 71 and affecting adversely on human bodies, which has also been a problem.
The first object of the invention is to solve foresaid problems and to provide a transferring belt device wherein recording sheet P advances properly onto transferring belt 71, uniform pre-transfer sheet charging is made, recording sheet P is attracted properly to transferring belt 71, thus uneven transferring is not created and less ozone is generated.
A charging device is well known wherein an elastic brush made of elastic conductive fiber materials is used as the charging means and is applied with an electric voltage. However, the tip of the elastic brush needs to be brought in contact with an object to be charged under a constant pressure. Accordingly, the elastic brush may collect dust on the belt, or may lose its elasticity and become curled after it has been used over a long period, causing uneven pressure contact and resulting in shorter life.
An object of the invention is to solve the above-mentioned problems and to provide an image forming apparatus being excellent in both transferability of a toner image and separability of recording sheets from a photoreceptor drum, wherein the apparatus is equipped with a transferring belt device capable of making the life of an elastic brush longer.
In case of the ambient condition for usage of high humidity, the resistance value between the ground and the apparatus is not infinity even when DC power source is turned off, resulting, therefor, in leakage of charges for transferring that easily takes place through transferring belt 71, recording sheet P and an elastic brush. Therefore, sufficient charges for transferring are not obtained, and transfer efficiency is lowered, and proper transferring can not be performed, which has been a problem.
The third object of the invention is to solve aforesaid problems with a simple constitution and to offer an image forming apparatus wherein uniform charging for transferring is made and characteristics of transferring and separation are uniform.